3-Hydroxyisobutyric aciduria is an inherited
metabolic disease caused by 3-hydroxyisobutyryl-CoA
dehydrogenase deficiency. Tissue accumulation and high urinary excretion of
3-hydroxyisobutyric acid is the biochemical hallmark of this disorder. Clinical phenotype is heterogeneous and generally includes dysmorphic features, delayed motor development, profound mental impairment, and acute
encephalopathy. Lactic acidemia is also found in the affected patients, indicating that
mitochondrial dysfunction may be involved in the pathophysiology of this disorder. Therefore, the aim of the present work was to investigate the in vitro effect of
3-hydroxyisobutyric acid (0.1, 0.5 and 1mM) on essential
enzymes of energy metabolism, namely the activities of the respiratory chain complexes I-V, total, cytosolic and
mitochondrial creatine kinase and Na(+), K(+)-
ATPase in cerebral cortex homogenates of 30-day-old rats. We also measured the rate of oxygen consumption in brain mitochondrial preparations in the presence of
3-hydroxyisobutyric acid.
3-Hydroxyisobutyric acid significantly reduced complex I-III (20%), without affecting the other activities of the electron transport chain. Furthermore,
3-hydroxyisobutyric acid did not change state III, state IV and the respiratory control ratio in the presence of
glutamate/
malate or
succinate, suggesting that its effect on cellular respiration was weak. On the other hand, the activities of total and
mitochondrial creatine kinase, but not cytosolic
creatine kinase, were inhibited (30%) by
3-hydroxyisobutyric acid. We also observed that
3-hydroxyisobutyric acid-induced inhibition of
mitochondrial creatine kinase activity was fully prevented by pre-incubation of the homogenates with
reduced glutathione,
alpha-tocopherol or the combination of
superoxide dismutase plus
catalase, suggesting that this inhibition was mediated by oxidation of essential
thiol groups of the
enzyme probably by
superoxide,
hydrogen peroxide and/or peroxyl radicals. It was also demonstrated that Na(+), K(+)-
ATPase activity from synaptic plasma membranes was markedly suppressed (37%) by
3-hydroxyisobutyric acid and that this effect was prevented by
alpha-tocopherol co-incubation implying that peroxyl radicals were probably involved in this action. Considering the importance of the affected
enzyme activities for brain metabolism homeostasis and neurotransmision, it is suggested that increased tissue levels of
3-hydroxyisobutyric acid may contribute to the neurodegeneration of patients affected by
3-hydroxyisobutyric aciduria and possibly explain previous reports describing elevated production and excretion of
lactate.